Implantable cardiac devices (“ICDs”) are well known in the art, and may operate to treat a variety of heart conditions. For example, included within the penumbra of ICDs are pacemakers, which may operate to maintain a healthy heart rate in a patient, or may operate to maintain a heart rate above a prescribed limit, such as, for example, to treat a bradycardia. Other examples of ICDs include cardioverter-defibrillators, which may operate to terminate potentially unhealthy arrhythmias or fibrillations. Some ICDs include both pacing circuitry and cardioverter-defibrillator circuitry. Other ICDs are designed to monitor a patient's heartbeat for diagnostic purposes.
Diabetes and heart disease share common risk factors, such as, for example, high blood pressure and cholesterol. Moreover, patients whose treatment for various heart conditions requires an ICD, are typically of an age group that is likely to develop diabetes during treatment with the ICD.
Abnormal cardiac rhythms, as measured by external electrocardiograms (“ECGs”), are known to be associated with diabetes. For example, T-wave oversensing in an ICD due to hyperglycemia has been reported. Specifically, it has been demonstrated that T-wave oversensing in the ICD was dependent on elevated serum glucose levels. Additionally, it has been reported that resting ECGs from young diabetics displayed prolonged corrected QT intervals.
Other studies have reported that ECGs showing flat or inverted T-waves were definitively associated with diabetes in otherwise healthy individuals. In fact, some reports suggest that there is an unusual prevalence of undetected diabetes among apparently healthy men who have non-specific T-wave abnormalities.
Even other studies have surmised that external ECGs could be used as an early marker for hypoglycemia because ECGs displayed a decrease in T-wave amplitude with declining blood glucose concentrations, while R-wave amplitudes remained unchanged. Accordingly, external ECGs facilitated observations that the R:T-wave ratio increased progressively in diabetic patients as blood glucose concentrations dipped to subnormal and hypoglycemic levels. It has been suggested that changes in external ECGs could be useful for detecting hypoglycemia and that a portable ECG recording system programmed with an algorithm capable of recognizing abnormal ECG patterns could alert a patient to check blood glucose levels if the measured R:T-wave ratio falls outside a pre-defined range.
Moreover, a system and method for monitoring diabetes-related blood constituents has been described. Specifically, the system provides a sensor that infers changes in blood glucose levels based upon external ECG signals and analyzes the external ECG signals to determine whether a group of parameters, including QRS, T-wave absolute mean, RMS values, QRS intervals, QT intervals, and RR intervals, is indicative of decreased blood glucose and/or insulin levels.
Accordingly, it would be advantageous to develop an ICD that, in addition to treating a particular heart condition, also is capable of continuously monitoring the patient's heartbeat for abnormal cardiac signals indicative of diabetes. Even more advantageous would be an ICD capable of monitoring at least one characteristic from internal cardiac signals, as opposed to monitoring external ECGs. Still more advantageous would be an ICD capable of monitoring multiple characteristics of the heartbeat to more accurately assess whether the patient has an abnormal heartbeat indicative of the early onset of diabetes and further capable of alerting the patient and physician of detected abnormalities as early as possible to increase the chances of effective diabetes treatment.